4,512 research outputs found

### Comment on "Phonon Spectrum and Dynamical Stability of a Dilute Quantum Degenerate Bose-Fermi Mixture

We show that the conclusions of a recent PRL by Pu et al is incorrect.Comment: late

### Casimir Force between a Small Dielectric Sphere and a Dielectric Wall

The possibility of repulsive Casimir forces between small metal spheres and a
dielectric half-space is discussed. We treat a model in which the spheres have
a dielectric function given by the Drude model, and the radius of the sphere is
small compared to the corresponding plasma wavelength. The half-space is also
described by the same model, but with a different plasma frequency. We find
that in the retarded limit, the force is quasi-oscillatory. This leads to the
prediction of stable equilibrium points at which the sphere could levitate in
the Earth's gravitational field. This seems to lead to the possibility of an
experimental test of the model. The effects of finite temperature on the force
are also studied, and found to be rather small at room temperature. However,
thermally activated transitions between equilibrium points could be significant
at room temperature.Comment: 16 pages, 5 figure

### Surface-atom force out of thermal equilibrium and its effect on ultra-cold atoms

The surface-atom Casimir-Polder-Lifshitz force out of thermal equilibrium is
investigated in the framework of macroscopic electrodynamics. Particular
attention is devoted to its large distance limit that shows a new, stronger
behaviour with respect to the equilibrium case. The frequency shift produced by
the surface-atom force on the the center-of-mass oscillations of a harmonically
trapped Bose-Einstein condensate and on the Bloch oscillations of an ultra-cold
fermionic gas in an optical lattice are discussed for configurations out of
thermal equilibrium.Comment: Submitted to JPA Special Issue QFEXT'0

### Thermal van der Waals Interaction between Graphene Layers

The van de Waals interaction between two graphene sheets is studied at finite
temperatures. Graphene's thermal length $(\xi_T = \hbar v / k_B T)$ controls
the force versus distance $(z)$ as a crossover from the zero temperature
results for $z\ll \xi_T$, to a linear-in-temperature, universal regime for
$z\gg \xi_T$. The large separation regime is shown to be a consequence of the
classical behavior of graphene's plasmons at finite temperature. Retardation
effects are largely irrelevant, both in the zero and finite temperature
regimes. Thermal effects should be noticeable in the van de Waals interaction
already for distances of tens of nanometers at room temperature.Comment: enlarged version, 9 pages, 4 figures, updated reference

### Anomalous galvanomagnetism, cyclotron resonance and microwave spectroscopy of topological insulators

The surface quantum Hall state, magneto-electric phenomena and their
connection to axion electrodynamics have been studied intensively for
topological insulators. One of the obstacles for observing such effects comes
from nonzero conductivity of the bulk. To overcome this obstacle we propose to
use an external magnetic field to suppress the conductivity of the bulk
carriers. The magnetic field dependence of galvanomagnetic and electromagnetic
responses of the whole system shows anomalies due to broken time-reversal
symmetry of the surface quantum Hall state, which can be used for its
detection. In particular, we find linear bulk dc magnetoresistivity and a
quadratic field dependence of the Hall angle, shifted rf cyclotron resonance,
nonanalytic microwave transmission coefficient and saturation of the Faraday
rotation angle with increasing magnetic field or wave frequency.Comment: 5 pages, 3 figures, version as publishe

### Vector and tensor perturbations in Horava-Lifshitz cosmology

We study cosmological vector and tensor perturbations in Horava-Lifshitz
gravity, adopting the most general Sotiriou-Visser-Weinfurtner generalization
without the detailed balance but with projectability condition. After deriving
the general formulas in a flat FRW background, we find that the vector
perturbations are identical to those given in general relativity. This is true
also in the non-flat cases. For the tensor perturbations, high order
derivatives of the curvatures produce effectively an anisotropic stress, which
could have significant efforts on the high-frequency modes of gravitational
waves, while for the low-frenquency modes, the efforts are negligible. The
power spectrum is scale-invariant in the UV regime, because of the particular
dispersion relations. But, due to lower-order corrections, it will eventually
reduce to that given in GR in the IR limit. Applying the general formulas to
the de Sitter and power-law backgrounds, we calculate the power spectrum and
index, using the uniform approximations, and obtain their analytical
expressions in both cases.Comment: Correct some typos and add new references. Version to be published in
Physical Reviews

### Topological phase transitions in ultra-cold Fermi superfluids: the evolution from BCS to BEC under artificial spin-orbit fields

We discuss topological phase transitions in ultra-cold Fermi superfluids
induced by interactions and artificial spin orbit fields. We construct the
phase diagram for population imbalanced systems at zero and finite
temperatures, and analyze spectroscopic and thermodynamic properties to
characterize various phase transitions. For balanced systems, the evolution
from BCS to BEC superfluids in the presence of spin-orbit effects is only a
crossover as the system remains fully gapped, even though a triplet component
of the order parameter emerges. However, for imbalanced populations, spin-orbit
fields induce a triplet component in the order parameter that produces nodes in
the quasiparticle excitation spectrum leading to bulk topological phase
transitions of the Lifshitz type. Additionally a fully gapped phase exists,
where a crossover from indirect to direct gap occurs, but a topological
transition to a gapped phase possessing Majorana fermions edge states does not
occur.Comment: With no change in text, the labels in the figures are modifie

### Kohn-Luttinger superconductivity in graphene

We investigate the development of superconductivity in graphene when the
Fermi level becomes close to one of the Van Hove singularities of the electron
system. The origin of the pairing instability lies in the strong anisotropy of
the e-e scattering at the Van Hove filling, which leads to a channel with
attractive coupling when making the projection of the BCS vertex on the
symmetry modes with nontrivial angular dependence along the Fermi line. We show
that the scale of the superconducting instability may be pushed up to
temperatures larger than 10 K, depending on the ability to tune the system to
the proximity of the Van Hove singularity.Comment: 5 pages, 3 figure

### Spin polarization and effective mass: a numerical study in disordered two dimensional systems

We numerically study the magnetization of small metallic clusters. The
magnetic susceptibility is enhanced for lower electronic densities due to the
stronger influence of electron-electron interactions. The magnetic
susceptibility enhancement stems mainly from an enhancement of the mass for
commensurate fillings, while for non-commensurate fillings its a result of an
enhancement of the Land\'e $g$ factor. The relevance to recent experimental
measurements is discussed.Comment: 4 pages, 4 figures, accepted for publication in PR

### Traversable wormhole in the deformed Ho\v{r}ava-Lifshitz gravity

Asymptotically flat wormhole solutions are found in the deformed
Ho\v{r}ava-Lifshitz gravity. It turns out that higher curvature terms can not
play the role of exotic matters which are crucial to form a traversable
wormhole, and external exotic sources are still needed. In particular, the
exotic matter behaves like phantom energy if Kehagias-Sfetsos vacuum is
considered outside the wormhole. Interestingly, the spherically symmetric
setting makes the matter and the higher curvature contribution satisfy
four-dimensional conservation of energy in the covariant form.Comment: 13 pages, 2 figures, version published in Phys. Rev.

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